Add background to Site Nav Bar

Hi there! 

I'm very new to squarespace and have little to no experience with coding. This combined with me being an illustrator and perfectionist can sometimes be a pain in the ass.

I've found a font that's too nice to get rid of just yet. The problem is that it's a bit messy and can be hard to read since the background is kindof messy aswell. So to compensate I'd like to add something of a background to just the Site Nav Bar.

Is this even possible or is it time for me to change into a better suited font? Attached is a picture of the current status. Below is what I'd love it to look like. 

@Arvtagaren This is possible but it might be a bit of trial and error to get the correct image size. Check out this tutorial from Becca at @inside_the_square: